1. Field of the Invention
The present invention relates to a solid state imager element composed of a CCD (change coupled device) suitable for use in a two-dimensional sensor (so-called imager), a linear sensor and so on.
2. Description of the Prior Art
A CCD solid state imager element used for a linear sensor, a two-dimensional sensor and so on is composed of a sensor portion 1, a read gate portion 3 having a gate electrode 2 and a register portion 5 having a transfer electrode 4 in a CCD structure, as shown in, for example, a potential diagram of FIG. 1. In this CCD solid state imager element, a signal charge 6 in the sensor portion 1 is read out, transferred to the register portion 5 through the read gate portion 3 and transferred through the register portion 5 to be finally outputted. Read-out and transfer of the signal charge in the example shown in FIG. 1 is performed such that the read gate portion 3 is turned on once to accummulate therein the signal charge 6 in the sensor portion 1 and then turned off to read out the signal charges 6 to the register portion 5. Alternatively, a signal read-out and transfer method as shown in FIG. 2 is also known where the read gate portion 3 is turned on to immediately read out the signal charge 6 in the sensor portion 1 to the register portion 5.
Generally, the output portion delivers outputs such that an output when no signal charge is transferred to the register 5 is determined to be a zero level.
Incidentally, in a linear sensor, the sensor portion 1 corresponds to a line sensor portion of the linear sensor while the register portion 5 to an output shift register portion disposed in parallel with the line sensor portion in the examples of FIGS. 1 and 2. Also, in a two-dimensional sensor, the sensor portion 1 corresponds to a sensor portion aligned in two dimension, and the register portion 5 to a vertical shift register portion disposed on one side of respective columns of the sensor portion for vertically transferring the signal charges from the sensor portion.
In the above-mentioned conventional solid state imager element, there arises a problem that unnecessary charges 6a or noise generated on the surface of the read gate portion 3, when the read gate portion 3 is off, that is, during light receiving periods, penetrates the sensor portion 1. For this reason, the noise generated in the read gate portion 3 is read out and transferred to the register portion 5 in addition to the signal charges generated in the sensor portion 1. Therefore, when a noise measurement is made in a normal condition (a difference between an output derived when no signal charge is transferred to the register 5 and an output derived when signal charges are read out of the sensor portion 1 and then transferred via the register portion 5 is measured in a dark place), the noise generated in the read gate portion 3 appears as it is as noise of the solid state imager element.